The agent of Borussia Dortmund striker Robert Lewandowski has claimed that interested parties must raise their offers if they are to sign his client.

The 23-year-old has been strongly linked with the Premier League following an impressive season in Germany, but Maik Barthel has insisted that no offers have met Dortmund's valuation.

"Unofficially there are offers from English top clubs. We respect the contract situation in Dortmund and don't want to take part in speculation. A £13m transfer fee is not suitable," Barthel told Goal.com.

The Pole scored 22 league goals for Dortmund last season.